High performance chirped electrode design for large area optoelectronic devices

ABSTRACT

An electro-optic device with a doped semiconductor base and a plurality of pixels on the semiconductor base, each pixel including: a multiple quantum well formed on the semiconductor base, an oppositely doped semiconductor layer on the multiple quantum well, and a top electrode on the semiconductor layer, the top electrode shaped to produce an approximately uniform lateral resistance in the pixel. An embodiment is a large area modulator for modulating retro-reflector systems, which typically use large area surface-normal modulators with large lateral current flow. Uniform resistance to each part of the modulator decreases location dependence of frequency response. A chirped grid electrode balances semiconductor sheet resistance and metal line resistance components of the series resistance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Non-Provisional of Provisional (35 USC 119(e))application 60/972,179 filed on Sep. 13, 2007, the entire disclosure ofwhich is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Technical Field

This application is related to electrical contacts or electrodes forlarge area electro-optic devices, and more specifically, for electricalcontacts for modulating retroreflectors, photodiodes, and other largearea optoelectronic devices.

2. Related Technologies

Large area surface-normal optoelectronic devices include modulatingretroreflectors (MRR) and large area photodiodes (PDs). Both MRRs andPDs can be designed either with or without focusing optics.

A modulating retroreflector developed by the Naval Research Laboratoryis described in U.S. Pat. No. 6,154,299 to G. Charmaine Gilbreath,Steven R. Bowman, William S. Rabinovich, Charles H. Merk, and H. E.Senasack, “Modulating retroreflector using multiple quantum welltechnology”, the entire disclosure of which is incorporated by referenceherein. The modulating retroreflector allows free-space opticalcommunication between a node with minimal power, weight, and pointingability (e.g., a UAV) and a node with higher power, weight, and pointingability (e.g. a ground station).

Modulating retroreflectors were demonstrated before the invention of thelaser, but were restricted to short distances and low data rates. Thefirst known description of a modulating retroreflector for free-spaceoptical communication is in Harry Stockman, “Communications by Means ofReflected Power,” Proceedings of the IRE, pp. 1196-1204, October 1948.Recent advances in optoelectronic devices and free-space optics havegreatly increased the capabilities of MRR systems. Examples arediscussed in “Peter G. Goetz, William S. Rabinovich, Rita Mahon, Mike S.Ferraro, James L. Murphy, H. Ray Burris, Mena F. Stell, Chris I. Moore,Michelle R. Suite, Wade Freeman, G. C. Gilbreath, and Steven C. Binari,“Modulating Retro-Reflector Devices and Current Link Performance at theNaval Research Laboratory,” MILCOM 2007, Orlando, Fla., October 2007.”

There are two basic classes of retro-reflectors, “cat's eye” and cornercube retro-reflectors. “Cat's eye” retro-reflectors combine lensesand/or mirrors and incorporate an optical focus. Several variations ofcat's eye retro-reflectors are described in in Mark L. Biermann, et al,“Design and analysis of a diffraction-limited cat's-eye retroreflector,”Opt. Eng. 41, 1655 (2002).

Corner cube retro-reflectors (CCRs) are nonfocusing. Tradeoffs betweenmodulating retro-reflectors of the corner cube type and the cat's eyetype are discussed in “Peter G. Goetz, William S. Rabinovich, RitaMahon, Lee Swingen, G. Charmaine Gilbreath, and James Murphy, “PracticalConsiderations of Retroreflector Choice in Modulating RetroreflectorSystems,” IEEE LEOS 2005 Summer Topicals, talk TuA3.5, San Diego,Calif., 25-27 Jul. 2005.

Gridded electrodes for use in solar photovoltaic cells are discussed inH. B. Serreze, “Optimizing Solar Cell Performance by SimultaneousConsideration of Grid Pattern Design and Interconnect Configuration,”1978. Conference Record of the IEEE Photovoltaic Specialists Conferencepp. 609-614, 13th IEEE PVSC, pp. 609-614, 1978, and in A. R. Burgers,“How to design optimal metallization patterns for solar cells”, Progressin Photovoltaics: Research and Applications; Vol. 7, No. 6, pp 457-461,1999.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cat's eye modulating retroreflector system.

FIG. 2 is an illustration of a chirped electrode in accordance with anaspect of the invention.

FIG. 3A is a top view of the chirped electrode of FIG. 2.

FIG. 3B is a side view of the chirped electrode of FIG. 2.

FIG. 4 illustrates one pixel of the chirped electrode of FIG. 2.

FIGS. 5A and 5B illustrate spacing between fingers of the grid electrodeshown in FIG. 4.

FIG. 6 illustrates a two-level chirped electrode grid in accordance withan aspect of the invention.

FIG. 7 illustrates the 20%-80% rise time and 20%-80% fall times as afunction of distance from the electrical wirebond contact for anexemplary modulating retroreflector.

FIGS. 8A and 8B illustrate a transmissive modulator with a reflectivemetallic top contact for each pixel that acts as both the electricalcontact and the mirror and extends over the entire pixel.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention are directed to modulators for modulatingretroreflectors, to photodiodes, and to other electro-optic devices.

The contact electrodes described herein are useful in electro-opticdevices including large area modulators with focusing optics and withoutfocusing optics, and large area photodiodes with focusing optics andwithout focusing optics. Use of the electrodes described herein canimprove performance of these devices, for uniformity of bandwidth, orfor the balance of lateral resistance with optical efficiency caused byshadowing of the device by the metal electrodes.

Note that only the epitaxially grown layers of the semiconductor layersare shown in these diagrams. Additional structure can be present, suchas a semiconductor wafer.

FIG. 1 illustrates a cat's eye modulating retroreflector 10, whichincludes lenses 11, a reflector 14, and an electro-optic modulator 20arranged in front of the reflector 14.

Cat's eye modulating retroreflectors allow increased data rates andranges compared to corner cube-based modulating retroreflectors, howeverthey are more complex. The use of focusing optics in cat's eyeretro-reflectors enables them to have large optical apertures withsmall, and thus fast, modulators. However, the location of the focalpoint on the mirror depends on the angle of incidence. Modulators aretypically placed at or very near the focal surface. As can be seen inFIG. 1, the angle of incidence determines the location of the focusedspot on the modulator 20. Large area devices provide a large field ofview (FOV), but only a small spot on the retroreflector's modulator 20is illuminated at any one time. For example, the illuminated spot canhave a diameter of about ten to a few hundred microns.

Use of a cat's eye retroreflector design imposes some designconsiderations on a modulating retroreflector. For example, mostmodulating retroreflectors to date use a large area, surface-normalmodulator. A flat focal plane most easily accommodates these flatmodulators and is required if the modulators are reflective. However,curved focal surfaces can produce the best results in terms of f/# andfield of view. This drives positioning of electrical contacts to theperiphery of the array, typically by wirebonding. Peripheral contactsare also used for reflective modulators on absorptive substrates, withillumination from the epitaxially grown side of the device. Peripheralcontacts can also be used with other types of modulators to simplifypackaging.

FIGS. 2, 3A, 3B, and 4 illustrate an exemplary electro-optic modulator20 that can be used in a cat's eye modulating retroreflector such as theFIG. 1 retroreflector system or in a corner cube retroreflector system.FIG. 3A is a top view of the modulator 20, and FIG. 3B is a crosssectional view of the modulator 20. The modulator 20 is formed of anarray of pixels, in order to take advantage of the small active areawhile still covering the entire field of view. Each pixel is larger thanthe expected spot size.

The same basic design could be used to make a wide field of viewphotoreceiver by replacing the modulator array with a photodiode array,which can be used as a matched optic to detect arrival angle tointelligently drive a modulator array, as described in “William S.Rabinovich, Peter G. Goetz, Rita Mahon, Lee Swingen, James Murphy,Michael Ferraro, H. Ray Burris, Jr., Christopher I. Moore, MichelleSuite, G. Charmaine Gilbreath, Steven Binari, and David Klotzkin,“45-Mbit/s Cat's Eye Modulating Retroreflectors,” Optical Engineering,vol. 46, no. 10, 104001, October 2007.”

In an exemplary embodiment, each pixel in the modulator 20 has a p-i-ndiode configuration, having an n-type semiconductor layer in contactwith an n-metal contact, an intrinsic region multiple quantum well, anda p-type semiconductor layer in contact with a p-metal electricalcontact. For example, FIG. 4 shows one pixel of the modulator 20, withan n-type semiconductor base 21, an n-metal contact 27, an intrinsicregion containing multiple quantum wells 28, and a p-type semiconductorlayer 29 in contact with a p-type metal electrical contact 30. It shouldbe noted that the intrinsic region of a photodiode would not necessarilycontain a multiple quantum well.

The pixels each have an n-type semiconductor base, n-metal electrode,multiple quantum well, top layer of p-type semiconductor material, andp-metal electrode. In the exemplary embodiment, each pixel may have itsown n-type electrodes or share a common n-type electrode. The electricalcontacts 27 and 30 are arranged so electrical contact with each pixel ismade on the periphery of the modulator. Each pixel can be drivenindependently.

The modulator 20 is arranged so interrogating light is surface-normal tothe modulator.

The top electrical contact 30 can be any metal or combination of metals,deposited on the p-semiconductor layer 29 by electron beam evaporationor another suitable technique. The intrinsic region 28 may contain amultiple quantum well consisting of alternating layers of epitaxiallygrown semiconductor. In an exemplary embodiment, each pixel has a topelectrical contact grid which is designed to make the lateral resistanceacross the pixel as small and as uniform as possible. For an arbitraryspot on a pixel, the lateral resistance R_(lateral)(x, y) isR_(lateral)(x, y)=R_(m)(x, y)+R_(sn)(x, y)+R_(sp)(x, y), where R_(m) isthe resistance of the metallic grid conductor, R_(sn) is the lateralsheet resistance of the n-type semiconductor layer, and R_(sp) is thelateral sheet resistance of the p-type semiconductor layer. Both R_(m)and R_(sn) increase with the distance from the electrical contact.

Alternative non-chirped modulator designs that have been measured haveshown a decrease in frequency response as the y distance from thewirebond/bondpad increases due to this increase in R_(m) and R_(sn). (Inorder to keep R_(lateral) as uniform as possible across the device,R_(m)+R_(sn) must be balanced by R_(sp).)

For the chirped-grid electrode modulator, the R_(sp) at a location isdetermined primarily by the distance from the location to the metal gridof the top electrode. The p-metal grid structure has a main busextending longitudinally along the center of the p-semiconductor layer,and a number of fingers extending perpendicularly to the main bus on thetop of the p-semiconductor layer. The spacing between fingers is largerin the region closest to the n-metal contact 27, and is smaller in theregion furthest from the n-metal contact 27. This decrease in spacingcauses a decrease in R_(sp), balancing the increase in R_(m)+R_(sn).

The pixelization of the modulator array provides a first level ofequalization of the lateral resistance, in addition to taking advantageof the small spot size of the retroreflector lens system.

The grid structure of the top electrical contact divides the individualpixels into “unit cells” for a second level of equalization of thelateral resistance R_(sp). In this embodiment, the unit cell is thefinal level of equalization, and the slowest part of the unit cell ismidpoint between the fingers, farthest from the main bus.

As illustrated in FIGS. 5A and 5B, equalization of the frequencyresponse between unit cells is accomplished with a variable finger pitchS(x). This creates a “chirped” design, in a manner similar to chirpeddesigns of diffraction gratings. Unit cells closest to the electricalcontact 27 have the largest finger pitch S(x), and the pitch decreasesas distance from the electrical contact 27 increases. By comparingpoints 1 and 2 in FIGS. 5A and 5B respectively, it is seen R_(m2) islarger than R_(m1) due to the distance from the electrode bond pad, andR_(sp2) is smaller than R_(sp1) due to the smaller spacing S(x) betweenfingers farther away from the bond pad. The sum of R_(m1)+R_(sp1) isnearly equal to the sum of R_(m2)+R_(sp2).

The finger location for the single bus design of FIG. 5 can becharacterized by the equation x_(n)=x₀+n*S_(max)−*[n(n−1)/2], forn=0,1,2, . . . , where x₀ is the position of the first finger, S_(max)is the maximum finger pitch, δ is the pitch decrement, and n is thefinger index. However, different spacing can be used, depending ondesign trade-offs.

An optimum finger pitch can be determined by the bus width, fingerwidth, and metal contact shadowing factor.

The highest density of fingers is far from the electrical contacts,making the optical transmission of the modulator lowest at normalincidence, for example, when the light hits the center of the array.However, the retroreflector optics have an opposite dependence on angle.The retroreflector optics have an optical return that is highest atnormal incidence. The net effect is a beneficial flattening of themodulating retroreflector optical return as a function of angle.

The main busbar can be tapered, with the widest part near the bondpad,and the narrowest part farthest from the bondpad or can haveapproximately the same width throughout its length.

For a given metal shadowing factor, the finger width WF is preferably asthin as technologically feasible. In addition, metal widths for thebusbar and fingers should be significantly smaller than the spot size inthe active area of a cat's eye MRR modulator or photodiode, in order toeliminate or minimize dead spaces. Therefore, large width busbars thatare used in some solar cell designs are not suitable for MRR modulators.Preferably, the MRR modulator electrodes include long, thin fingers andbusbars, and/or multiple busbars per pixels.

Wide peripheral contacts, such as those used in solar cell electrodedesigns, can be used in MRR modulator electrodes; however, theytypically are not optimized in terms of reduced resistance versus addedcapacitance of the shadowed inactive area required for the wideperipheral contacts.

It is preferred that the top contact metal is not reflective at theoperational wavelengths, to avoid the contact acting as a mirror in thecat's eye optic and retroreflecting unmodulated light back to theilluminating laser. One method for avoiding such shiny metal contacts isto electroplate the top contacts. Another method is to anneal the topcontact, to decrease the reflectance of the evaporated metal. Othermethods include using other absorptive or less reflective films to coverthe electrode grid and to prevent reflections.

One determination to make in grid design is whether to grow themodulator structure as p- or n-type contact on top. An importantfunction of the chirped grid design is to make the frequency responseacross the surface of the pixel as low and as uniform as possible.Increasing the thickness of the doped semiconductor contact layers, e.g.layers 21 and 24/29 in FIG. 3B, can reduce resistance to a point, butthe effect is not as large as produced by the chirped grid contactdesign of FIG. 5. Thick top contact layers would increase the mesaheight and add to processing complexity. In contrast, using a thicksemiconductor base as the bottom contact makes processing simpler byrelaxing the accuracy needed to etch to the bottom contact layer. Sincethe mobility of electrons is an order of magnitude greater than that ofholes, the wafers are preferably grown with the p-type contact as thetop contact. This allows the metal grid pattern to improve theperformance of the lowest mobility (p-type) contact; a thicker bottomcontact maintains low resistance in the n-type contact.

While FIGS. 5A and 5B illustrate a single degree of fingermetallization, the chirped electrode design can be extended to higherlevels of metallization, with the first level of fingers acting asbusbars for the next level of fingers.

FIG. 6 illustrates a pixel with a two-layer chirped grid electrodelayout. The p-type grid electrode 71 is in contact with the top p-dopedsemiconductor layer 72, and includes a bondpad 73, a main busbar 72extending in the x direction along the top of the semiconductor layer72, and a number of fingers 74-82 that extend in the y direction. Thefingers have a spacing pitch that decreases linearly with increasingdistance from the bondpad. To achieve another dimension of equalization,a series of perpendicularly arranged smaller fingers extend in the ydirection from each of the electrode fingers. The smaller fingers arealso chirped in design, with a spacing pitch that decreases withincreasing distance from the busbar 74. The busbar 74 can have aconstant width, or a tapered width that decreases with increasingdistance from the bondpad 73.

The device of FIGS. 2-5 can also act as a large-area photodiode withreduced lateral resistance, and the electrode design of FIG. 6 can beused in a large area photodiode.

Such photodiodes, are usually p-i-n, and may or may not include amultiple quantum well in the intrinsic region. The photodiode caninclude any semiconductor material, including Silicon.

While the exemplary embodiment shown in FIGS. 2-5 has a p-type topcontact and an n-type bottom, it is also envisioned that the top contactcan be an n-type and the bottom can be a p-type.

EXAMPLE

An 8 mm by 8 mm modulator with a chirped grid electrode was designed fora cat's eye modulating retroreflector. The device was segmented into a6×2 array of pixels. A chirped top contact was deposited using electronbeam evaporation.

In this example, the finger width WF for electron beam evaporation isabout 4 microns. However, the finger width can be wider or narrower,(e.g., a few microns). Electroplating could be used to allow narrowerwith thicker metal for the same or lower overall resistance.

The maximum width of the busbar was set at 22 microns, tapering down toabout 12 microns at the far end of each pixel. This busbar is wider thandesired optically, however, based on previous designs, an average metalcoverage of 2.3% was determined sufficient for the desired bandwidth yetstill optically acceptable. This metal coverage, the busbar geometry,and the minimum W_(F) set the number of fingers at eighteen, the maximumpitch S_(max) at 300 microns, and the pitch decrement δ at 10 microns.The finger pitch decreases linearly from near the bondpad to a minimumat the far end of the busbar.

This example modulator was tested on a transmissive stage. A fiberopticprobe was used to illuminate the modulator from above, and the light wascollected underneath into a 125-MHz photodetector. In this manner, thelight could be localized onto any area of the modulator.

Measurements were taken at the midpoint between the tips of the gridfingers for various values. The modulator was driven with a square waveusing a low impedance driver, and the 20%-80% rise and 80%-20% falltimes were measured using a digital oscilloscope with a filteredbandwidth of 150 MHz. The electrical drive signal going to the modulatorhad a measured rise time of 2.0 ns and fall time of 1.8 ns. Measurementresults in FIG. 8 show that good uniformity of rise and fall times isachieved. Variations of approximately 5% in rise and fall time across afour millimeter long modulator device were measured. By comparison,alternative designs with a pixelated modulator with uniformly spacedelectrode grids can have variations in rise and fall times of up to300%.

Of particular interest is the decrease in rise time with increasingdistance from the wirebond. This improvement in risetime due toovercompensation of the increasing R_(m)+R_(sn) clearly demonstrates theutility of the chirped grid design in compensating for accumulation oflateral resistances.

Modulators used in modulating retroreflector systems are typically largearea surface normal devices with large current flow. Frequency responseis limited by the resistance-capacitance (RC) time constant. In thecat's eye modulating retroreflector systems with focusing optics, only asmall part of the total modulator area is illuminated at a time. Theuniform resistance to each part of the modulator provided by thepixelated, chirped electrode designs herein decreases locationdependence of the modulator's frequency response. Location dependence ofthe modulator's frequency response causes angle dependence of frequencyresponse in cat's eye MRRs.

Advantages of the pixelated, chirped electrode grid MRRs discussedherein include the ability to locate the electrical contacts at theperiphery of the modulator to simplify wiring and increase pixel densityand low and uniform lateral resistance, which produces a fast anduniform frequency response across the entire modulator area.

Moreover, eliminating the use of more metal than necessary in parts ofthe modulator that do not require it can reduce production costs. Inaddition, certain failures in epitaxially-grown semiconductor deviceshave been associated with metal coverage of pit defects, resulting inshorts across the device. These shorts are primarily of concern withlarge area devices such as MQW modulators. Eliminating unnecessarymetallization can decrease the probability of such shorts, which isdirectly related to the metal area coverage, and thus can increaseyield.

Embodiments of the invention are also directed to modulatingretro-reflector systems which do not use focusing optics, and tolarge-area photodiodes. One example of a MRR without focusing optics isa corner-cube retro-reflector.

Modulators in modulating retroreflectors and large-area photodiodes thatdo not use focusing optics can also suffer from the above-mentioned lackof uniformity in lateral resistance. Although these devices are notangle-dependent, modulators with uniformly spaced metal grids are notoptimized to balance the shadowing effect of the metal grid with thelateral resistance reduction provided by the metal grid, so differentareas of these devices can operate at different speeds. Some designsprovide excessive metallization in areas of the device to increasespeeds, leading to additional shadowing and lower optical efficiency.Large area photodetectors can also suffer from lack of uniformity infrequency response.

FIGS. 8B and 8A illustrate a modulator 110 with a transmissivesubstrate, suitable for use in a flat focal plane cat's eyeretroreflector. The modulator shown is configured in a 2×4 pixel array,although different number of pixels is also suitable. The pixels canalso be non-rectangular, for instance a circular modulator array can bemade up of pie -shaped pixels. The reflective metallic top contact 111acts as both the electrical contact and the mirror. The contact extendsover the entire pixel 112. In each pixel, the reflective metallic topcontact is layered over the i-type multiple quantum well 113, which isin contact with the common base semiconductor layer 114. The modulatoris mounted so the light 115 enters the modulator from the substrateside. The substrate can be thinned and polished to minimize absorption.The metal thickness would then avoid significant variations of frequencyresponse within a pixel.

Interior pixels can be electrically contacted with either wirebonding orflip chip bonding without obscuring the optical path. Peripheral contactis no longer necessary, so optical considerations need not be consideredin the location of the contact, and contact location can be optimizedsolely on electrical considerations.

An advantage of this modulator is that there is no need for a separatereflector, such as the reflector 13 of FIG. 1.

Drawbacks of this arrangement include making heatsinking of the devicemore difficult, as neither surface of the modulator is available fordirect contact with a heat sink. It also has the potential of loweringon-wafer yield, as the metal area is greatly increased, and any pitdefects in the epitaxy which extend beyond the top contact could resultin shorted devices. Optically, this method also puts the activemodulator layers as close as possible to the focal surface, whichminimizes the spot size, and can concentrate the optical power more thanwith the chirped electrode designs discussed above. A decrease inmodulator contrast ratio has been observed with increased optical powerdensity, which could limit this method's utility with high opticalpowers such as are used in long distance applications.

Note that the FIGS. 8A-8B device can also act as a photodiode, with orwithout multiple quantum wells.

In the modulators and photodiodes described above, the size of thepixels is a design tradeoff between complexity and speed. The diameterof the optical spot on the modulator array is can be between about 10 to100 μm, which gives a lower bound to the useful pixel size.

These examples include systems with multiple pixels. However,optoelectronic systems in accordance with other embodiments of theinvention are not pixelated or have only one pixel.

Although this invention has been described in relation to the exemplaryembodiments thereof, it is well understood by those skilled in the artthat other variations and modifications can be affected on the preferredembodiment without departing from scope and spirit of the invention asset forth in the claims.

1. An optoelectronic device comprising: a doped semiconductor base; andat least one pixel formed on the base, each pixel including: anoppositely doped semiconductor layer, and a top electrode formed on theoppositely doped semiconductor layer, wherein the top electrode isconfigured in a grid pattern with at least one busbar and a plurality offingers extending from the busbar, and spacing between the fingersdecreases with distance from the bondpad along the busbar.
 2. The deviceaccording to claim 1, further comprising: a layer of intrinsicsemiconductor formed between the doped semiconductor base and theoppositely doped semiconductor layer.
 3. A device according to claim 2,wherein the intrinsic semiconductor layer includes a multiple quantumwell.
 4. A device according to claim 3, wherein the device is amodulator for a modulating retroreflector.
 5. The device according toclaim 4 in combination with focusing optics and a reflector, wherein thedevice is positioned between the focusing optics and the reflector, ator near a focal plane of the focusing optics.
 6. The device according toclaim 4 in combination with a reflector, wherein interrogating light isreceived at the device, is modulated by the modulator, and is reflectedtoward a source of the interrogating light.
 7. The device according toclaim 1, wherein the device is a photodiode.
 8. A device according toclaim 1, wherein the doped semiconductor base is an n-type semiconductormaterial and the oppositely doped semiconductor layer is a p-typesemiconductor layer.
 9. The device according to claim 8, wherein the topelectrode is a p-metal electrode, and further including: an n-metalelectrode on the n-type semiconductor base.
 10. The device according toclaim 1, wherein the electrode spacing decreases linearly with distancefrom a bondpad along the busbar of the top electrode.
 11. The deviceaccording to claim 1, wherein the fingers extend from opposite sides ofthe busbar.
 12. The device according to claim 1, wherein the shape ofthe top electrode produces an approximately uniform lateral resistancethroughout the pixel.
 13. The device according to claim 1, wherein theat least one pixel formed on the base includes a plurality of pixels.14. An optoelectronic device comprising: a doped semiconductor base; andat least one pixel formed on the base, each pixel including: anoppositely doped semiconductor layer, an intrinsic region including amultiple quantum well formed between the base and the oppositely dopedsemiconductor layer, and a top electrode formed on the oppositely dopedsemiconductor layer, wherein the top electrode is a plurality ofparallel conductive fingers in electrical communication with each other,wherein the spacing between the fingers decreases with distance from oneedge of the pixel.
 15. An optoelectronic device comprising: atransparent doped semiconductor base; and at least one pixel formed onthe base, each pixel including: an oppositely doped semiconductor layer,and a top electrode formed on the oppositely doped semiconductor layer,being a reflective conductive material, and covering the oppositelydoped semiconductor layer of the pixel.
 16. The device according toclaim 15, further comprising: a layer of intrinsic semiconductor formedbetween the doped semiconductor base and the oppositely dopedsemiconductor layer.
 17. A device according to claim 15, wherein theintrinsic semiconductor layer includes a multiple quantum well.
 18. Adevice according to claim 15, wherein the device is a modulator for amodulating retroreflector.
 19. The device according to claim 18 incombination with focusing optics, wherein the device is positioned at ornear a focal plane of the focusing optics, wherein interrogating lightis received at the device, is modulated by the modulator, and isreflected toward a source of the interrogating light.
 20. The deviceaccording to claim 15, wherein the device is a photodiode.
 21. A deviceaccording to claim 15, wherein the doped semiconductor base is an n-typesemiconductor material, and the oppositely doped semiconductor layer isa p-type semiconductor layer.
 22. The device according to claim 21,wherein the top electrode is a p-metal electrode, and further including:an n-metal electrode on the n-type semiconductor base.
 23. The deviceaccording to claim 15, wherein the top electrode produces anapproximately uniform lateral resistance in the pixel.
 24. The deviceaccording to claim 15, wherein the at least one pixel comprises aplurality of pixels.